1. Field of the Invention
The present invention relates to the detection of chlorinated aromatic compounds in aqueous fluids. More particularly, the present invention relates to a method for measuring the concentration of chlorinated aromatic compounds in an aqueous fluid, and a composition and fiber optic probe for measuring that concentration.
2. Discussion of Background
Fiber optic probes for detecting various substances are well known. Such devices, whether intended for continuous remote operation or occasional laboratory analyses, are typically a solution containing an indicator responsive to the presence of the substance of interest (analyte). The presence of the analyte results in a change in the optical properties of the indicator, such as its color, the intensity of light it transmits, and its fluorescence emission spectrum. There is a wide selection of indicators for detection and analysis of corresponding analytes. For example, indicators are available for the qualitative and quantitative detection of oxygen, carbon dioxide, hydrogen ions (pH), glucose, ammonia, and certain metal ions.
Fiber optic-based detecting systems using spectrometric analysis techniques generally include a light source, a sample cell formed to admit a fluid containing the analyte of interest, an indicator, optic fibers for transmitting light to and from the sample cell and a light analyzer such as a spectrophotometer. Light is transmitted through the sample cell and received by the spectrophotometer, which reveals the spectrum of the received light. Fiber optic light transmitting fibers are favored in devices for remote operation. Further, a plurality of optical probes may be connected to a single light source and detector, as in the remote multi-position information gathering system described by Hirschfeld (U.S. Pat. No. 4,577,109).
Numerous methods exist for securing the indicator within such a probe. The indicator is usually immobilized in a polymer matrix by absorption or adsorption to form a composition. Preferably, the indicator is bound to the matrix so that the composition may be in contact with the fluid without loss of the indicating substance. The polymer matrix is preferably permeable to the fluid so that the analytes can reach the active sites bearing the indicator molecules. In general, the faster the fluid permeates the matrix, the shorter the response time of the probe.
The sensitivity and specificity of the probe are determined in part by the type and amount of the indicator present in the matrix. Preferably, the indicator is sensitive to the analyte but not to other chemical species the fluid might contain. The matrix is substantially optically transparent in the wavelength range for fluorescent excitation and emission of the indicator.
Specific examples of fluorescence indicators include U.S. Pat. No. 5,037,615, issued to Kane for a sensor unit with a fluorescence energy transfer indicator for measuring pH, oxygen and carbon dioxide concentrations, and metal ions. The indicator is contained in a porous glass plug at the distal (working) end of an optical fiber. Kritzman, et al. (U.S. Pat. No. 5,045,282) show a sensor having a substantially nonporous glass tip fused to or integral with an open end of glass optical fiber. A fluorescent substance is adsorbed on the internal surfaces of the pores; the tip is coated with a porous polymeric film prior to use. Suitable fluorescent materials include coumarin derivatives (pH and CO.sub.2 determination), and pyrene and other polycyclic aromatic compounds (O.sub.2 determination). Yafuso, et al. (U.S. Pat. No. 5,081,041) disclose a two-layer sensor for detecting hydrogen or hydroxyl ions (pH) in fluids. The sensor includes a fluorescence and pH indicator such as hydroxypyrene trisulfonic acid or derivatives thereof. Cox, et al. (U.S. Pat. No. 5,034,189) immobilize a fluorophor in a permeable, transparent polymeric matrix. When the matrix is exposed to the fluid being analyzed, the intensity of the fluorescent radiation is proportional to the concentration of the analytes in the fluid. For analyzing oxygen, 9,10 diphenyl anthracene is incorporated into a poly (dimethyl siloxane) or silicone matrix. For analyzing glucose concentration, 9,10-diphenyl anthracene in a poly (hydroxyethyl methacrylate) is used. Rhum, et al. (U.S. Pat. No. 5,019,350) disclose fluorescent polymers for the determination of blood gas or pH. Fluorescent organic substituents are covalently bonded to an organic, water-insoluble polymer through ester or amide linkages. Suitable polymers include hydroxyethyl methacrylate homopolymer, polyvinyl alcohol, and so forth. Suitable fluorescent indicators for blood oxygen include the pyrene butyric acids; indicators for pH include 4-(carboxymethyl) umbelliferone (CMU).
Despite the existence of fiber optic probes using well known indicators, it is believed that no detector exists for effectively detecting small concentrations of chlorinated aromatics, including polychlorinated biphenyls (PCBs). The detection of low levels of PCBs, particularly in groundwater, is a growing concern in environmental monitoring, thus, there is an immediate need for detectors of this kind. A satisfactory detector for the detection and measurement of chlorinated aromatics, including PCBs, should be sensitive to trace amounts (1 ppb or less), have a short response time, long-term stability and reproducibility, and be chemically inert to the operating environment.